Add-subtract counter

ABSTRACT

An add-subtract counter for preventing miscounts which might occur when add and subtract pulses are applied slightly out of phase to the counter, including a stop member having a plurality of stopping surfaces, each surface engaging the ratchet wheel during a different portion of the pawl stroke.

United States Patent [72] Inventors (ientiel De (iryse East Moline; Frederick E. Erickson, Port Byron; Gary D. Fredell, East Moline, Ill. [21 1 Appl. No. 574,707 [22] Filed Aug. 24, 1966 [45] Patented Feb. 2, 1971 [73] Assignee Gulf & Western Industries New York. N.Y., a corporation of Delaware. by mesne assignment [54] ADD-SUBTRACT COUNTER 4 Claims, 8 Drawing Figs.

[52] US. Cl 235/92, 340/379 [51] Int. Cl G06m 3/12 [50] Field of Search 235/92; 340/379; 74/126, 142

[5 6] References Cited UNITED STATES PATENTS 2,973,899 3/1961 Hegner 235/92 2.465. I 98 3/1949 Christiansen 74/ 1 42 2.620873 12/1952 Eaton 164/20 2.787.912 4/1957 Smallpeice 74/142 3.097.278 8/1963 A1derman..... 200/105 15 89.663 6/1926 Rogers 74/126 3,329,032 7/1967 Reich 74/126 2,540,808 2/1951 Bliss 235/92 2,668,012 2/1954 Lindeman 235/92 2,377,583 6/1945 Smith 74/126 2,426,715 9/1947 Tatter..... 74/143 2,522,734 9/1950 Wood 235/1 Primary Examiner-Maynard R. Wilbur Assistant ExaminerRobert F. Gnuse AttorneyMeyer, Tilberry and Body ABSTRACT: An add-subtract counter for preventing miscounts which might occur when add and subtract pulses are applied slightly out of phase to the counter, including a stop member having a plurality of stopping surfaces, each surface engaging the ratchet wheel during a different portion of the pawl stroke.

WENITED in 2mm SHEET 1 0F 3 EXTERNAL PULSERS y no. "3 f SUBTRACT SOL.

N S mm md E O S N WYL 8 R NRRL 0.0 O EEGE T V .ED T WE E A mMm I RED El DTY 7 wmm Fee w, YM B PATENTED FEB 2 I97! SHEEI 2 [IF 3 INVENTORS. FREDERIC K E. ERICKSON, GENTI EL M. DEGRYSE a GARY D. FREDELL BY Maya, 7% 8 Body FIG. 5

ATTORNEYS PATENTEUFEB 2|9n 3560.720

SHEET 3 [IF 3 INVENTORS. FREDERICK E. ERICKSON,

TIEL M. DEGRYSE 8 G Y D. FREDELL ATTORNEYS ADD-SUBTRACT COUNTER occur when add and subtract input pulses are applied slightly out of phase to the counter.

The invention is particularly applicable in conjunction with preventing miscounts by an add-subtract counter and will be described with particular reference thereto, although, it will be appreciated that the invention has broader applications.

Add-subtract counters are well known and are used, for example, in maintaining a continuous running count of the number of objects which have entered a storage space less the number of objects which have exited from the storage space. Generally, such counters include an'add solenoid and a subtract solenoid for respectively receiving add input pulses and subtract input pulses. The plunger of each solenoid serves to drive a shaft, which carries a numeral wheel, in opposing directions through a differential mechanism. The shaft is driven one step in a first direction in response to each receipt of an add input pulse and a like distance in an opposite direction in response to each receipt of a subtract input pulse. If both the input add and subtract pulses are received exactly in phase the differential will prevent rotation of the shaft so that the two pulses are canceled. However, if the two pulses are received slightly out of phase a miscount may result.

The problem of a miscount by an add-subtract counter may be explained by reference to the U.S. Pat. to I-Iegner No. 2,973,899 which describes a typical add-subtract counter. Hegners counter serves to add input add pulses and subtract input subtract pulses and display the result by means of a numeral wheel carried by a driven shaft, adapted for step by step rotation in opposing directions. The counter includes an add ratchet wheel and a subtract ratchet wheel which are respectively driven step by step in opposing directions by an add pawl and a subtract pawl. A bevel gear differential drivingly couples the two ratchet wheels with the shaft for driving the shaft in the correct direction. Both the add pawl and the subtract pawl are U-shaped, each having first and second ratchet wheel driving legs. Each pawl is pivotally mounted and is resiliently biased so that the second leg is normally in engagement with a ratchet wheel to prevent rotation of the wheel in a direction opposite to that when driven by the pawl. In operation, upon receipt of an appropriate add or subtract pulse, a solenoid is momentarily energized causing its associated add or subtract pawl to pivot so that the second leg is displaced from the ratchet wheel and the first leg is pivoted into engagement with and drives the ratchet wheel an initial portion of a one step movement. At the termination of the initial portion, the pawl returns toward its normal position by resilient force provided by a return spring so that the second leg engages and drives the ratchet wheel the remaining portion of the one step movement. During the period that the pawl is initially displaced from its nonnal position until the point in time that the first leg engages the ratchet wheel there is a short period of time during which the ratchet wheel is free floating. That is, the ratchet is not engaged by either leg of the pawl and may be driven at least a slight arcuate distance in either direction. In the event that, for example, the subtract ratchet wheel is driven, when in its free floating condition, by some external means or through the differential by the add pawl for a slight distance in the direction opposite from that which occurs when it is driven by its associated subtract pawl, a miscount will occur. That is, when the first leg of subtract pawl reaches the position where it would normally engage one tooth of the subtract ratchet wheel to drive the wheel through the initial portion of a one-step movement, that tooth has been displaced by a slight distance sufficient that the first leg of the pawl will miss the tooth, resulting in a miscount.

The present invention is directed toward an improved addsubtract counter having means to positively prevent a miscount, thereby overcoming the noted disadvantages, as well as others, of previous add-subtract counters.

The present invention contemplates that the add-subtract counter serves to add input add pulses and subtract input subtract pulses and display the result by means of a numeral wheel carried by a driven shaft-adapted for step by step rotation in opposing directions. It is further contemplated that the counter include an add ratchet wheel and a subtract ratchet wheel respectively driven step by step in opposing directions by an add pawl and a subtract pawl, and differential means drivingly coupling the ratchet wheels with the shaft.

In accordance with the present invention the improved addsubtract counter includes means for continuously preventing rotation of each ratchet wheel in a direction opposite to that obtained when the wheel is driven by its associated pawl.

In accordance with a more limited aspect of the present invention the rotation prevention means includes an add wheel stop member and a subtract wheel stop member respectively maintained in continuous rotation prevention engagement with the add and subtract ratchet wheels for preventing rotation thereof in directions opposite to their respective pawl driven directions.

Still further inStill further in accordance with the present invention, each stop member is adouble check stop member having a first stop surface for rotation prevention engagement with its associated ratchet wheel during the period that the pawl is being displaced from a normal position to a position wherein a first portion thereof engages and drives the ratchet wheel a first portion of a one step movement and a second stop surface for rotation prevention engagement with the associated ratchet wheel during the period that the pawl returns towards its normal position until a second portion of the pawl engages the ratchet wheel and drives it the remaining portion of the one step movement.

The primary object of the present invention is to provide an improved add-subtract counter which will not miscount rcgardless of the phasing of received add and subtract input pulses.

A still further object of the present invention is to provide an add-subtract counter which is simple in construction and economical to manufacture.

A still further object of the present invention is to provide an improved add-subtract counter which will have a long operating life time and which is fail proof during operation so as to continuously provide a correct indication of the difference in the number of received add and subtract input pulses.

The foregoing and other objects and advantages of the invention will become apparent from the following description of the preferred embodiment of the invention as read in connection with the accompanying drawings in which:

FIG. 1 is a plan view illustrating the preferred embodiment of the invention;

FIG. 2 is an elevational view of the embodiment of the invention illustrated in FIG. 1;

FIG. 3 is an enlarged partial view illustrating one aspect of the operation of the invention;

-FIG. 4 is an enlarged view similar to that of FIG. 3 illustrating a second aspect of the operation of the invention;

FIG. 5 is a partial view of FIG. 4 illustrating a still illustrating a still further aspect of the operation of the invention;

FIG. 6 is a schematic illustration of the electric circuit used in conjunction with the invention;

FIG. 7 is an enlarged view similar to similar to that shown in FIG. 3 illustrating the operation of the mechanism which results in the absence of a double check stop member; and,

FIG. 8 is a view similar to that shown in FIG. 7 illustrating the operation which obtains when double check stop members are provided.

GENERAL DESCRIPTION Referring now to the drawings and more particularly to FIGS. 1 and 2 there is illustrated the preferred embodiment of the invention wherein the add-subtract counter generally comprises a numeral wheel assembly A carried by a shaft B which is driven step by step through a differential assembly C by an add ratchet wheel D and a subtract ratchet wheel E. Ratchet wheels D and E are driven step by step in opposing directions by an add pawl F and a subtract pawl G, respectively, which in turn are respectively actuated by an add solenoid H and a subtract solenoid I. Ratchet wheels D and E are respectively prevented from rotating in directions opposite to that obtained when driven by their respective pawls F and G by means of an add wheel stop member .I and a subtract wheel stop member K.

As shown in FIGS. I and 2 the counter includes a U-shaped support frame having a base 12 and upstanding side legs 14 and 16 which terminate in outwardly directed front face mounting flanges l8 and 20, respectively. In addition, a plate 22 is secured to base 12 of fame 10 and extends perpendicularly therefrom in an upward direction and rearwardly in a direction parallel to upstanding side legs 14 and 16.

NUMERAL WHEEL ASSEMBLY Shaft B, which carries numeral wheel assembly A, extends perpendicularly between and is journaled for ration rotation in upwardly extending side legs 14 and 16. A gear 24 is fixed to shaft B. As best shown in FIGS. 2 and 3, gear 24 meshes with output gear 68 of the differential assembly C for driving shaft B in either of two opposing directions. A digit numeral wheel 28 is also fixed to shaft B and displays ten digits from 0 to 9, on its periphery. These digits are arranged in numerical order on wheel 28 so that for each discrete step movement of shaft B a different digit is displayed, depending on the direction of rotation. A second digit wheel 30 is also fixed to shaft B for rotation therewith. Wheel 30 however, is angularly adjusted so that the digit displayed is always one count greater than that displayed by wheel 28. A l0 numeral wheel 32 associated with digit numeral numeral wheel 28 is loosely mounted on shaft B on the left side (as viewed in FIG. 2) of wheel 28. Similarly, a second ten wheel 34 associated with wheel 30 is loosely mounted on shaft B on the left side (as viewed in FIG. 2) of wheel 30. Interposed between numeral wheel 28 and numeral wheel 32 there is provided tens transfer means including a gear 36 loosely mounted on shaft 38 extending between side legs 14 and 16. Gear 36 includes four equally spaced wide teeth 40. Interposed between adjacent wide teeth 40 there is provided one of four equally spaced narrow teeth 42. The narrow teeth 42 serve to engage gear 44 on the right-hand side (as viewed in FIG. 2) of numeral wheel 32. The wheel teeth serve once during each cycle of rotation of digit numeral wheel 28 to engage both gear 44 and a slot (not shown) on the left-hand side of digit wheel 28 so that as wheel 28 completes one revolution it drives wheel 32 one step to complete the tens transfer. Similar transfer means are interposed between numeral wheels 30 and 34 and accordingly like numerals are shown for identifying like components.

lnterposed between numeral wheels 30 and 32 there is provided acam wheel 46 which is fixed to shaft B for rotation therewith. This cam wheel includes an indentation 48 (see FIG. 1) in its periphery. Located immediately to the right (as viewed in FIG. 2) of wheel 46 there is provided a second cam wheel 50 which is loosely mounted on shaft B and is connected by means of a pin (not shown) to numeral wheel 32 so that cam wheel 50 rotates in the same manner as numeral wheel 32. This cam wheel also includes an indentation 52 (see FIG. 2) in its periphery. Single pole double throw switches SW-2 and SW-3 are mounted between plate 22 and leg 14 and are provided with a common spring biased cam follower 54 which is normally in engagement with the periphery of cam wheels 46 and 50. Whenever numeral wheels 28 and 32 display a 00 indication, cam wheel indentations 52 and 54 are in alignment and receive cam follower 54, actuating both switches SW-2 and SW-3. To the left of numeral wheel 34 left of numeral wheel 34 (as viewed in FIG. 2) there is provided another cam wheel 58 fixed to shaft B. This cam wheel ineludes an indentation 60 in its periphery. Immediately to the right of wheel 58 there is provided a second cam wheel 62 which is loosely mounted to shaft B and is connected by means of a pin (not shown) to wheel 34 so that it rotates in the same manner as does wheel 34. This cam wheel also includes an in dentation 64 in its periphery. Immediately below cam wheels 58 and 62 there is provided a single pole double throw switch SW-l which is mounted between leg 14 and plate 22 and is provided with a spring biased cam follower 66 which is biased into engagement with the periphery of wheels 58 and 62. Whenever numeral wheels 34 and 30 display a 00 indication, cam wheel indentations 60 and 64 are in alignment and receive cam follower 66, actuating switch SW-l.

DIFFERENTIAL ASSEMBLY AND RATCHET WHEELS Differential assembly C is mounted between plate 22 and upstanding leg 16. This differential assembly preferably takes the form of a standard spur gear differential or, if desired, may take the form of the bevel gear differential constructed as illustrated in the US Pat. to M. Hegner No. 2,973,899. Briefly, the differential assembly C includes an output gear 68 which meshes with gear 24 for purposes of driving shaft B. A pair of star-shaped ratchet wheels D and E provide the two required inputs on opposite sides of the differential assembly C. The star wheels D and E preferably take the form as shown for example with respect to wheel E in FIGS. 3. 4, 5, 7 and 8. Each star wheel includes ten equally spaced teeth. As is well known to those skilled in the art, if star wheel D is driven one step in a given direction and star wheel E is held stationary then output gear 68 of the differential will be displaced one step in the direction of rotation in which star wheel D is driven. Also, if star wheel D and star wheel E are both driven simultaneously in opposing directions by the same amount the output gear 68 will remain stationary.

RATCHET WHEEL DRIVE PAWLS Pawls F and G serve to respectively drive star wheels D and E in step by step movement in opposing directions. Pawls F and G are in turn actuated by solenoids H and I, respectively. Pawls F and G each take the form of a substantially U-shaped member having a first leg 70 and a second leg 72 separated by an intermediate portion 74 (see FIG. 4). The intermediate portions 74 of the two pawls F and G are loosely mounted on a common pivot post 76 which extends between plate 22 and leg 16. Pawl G is normally resiliently biased in a clockwise direction about post 76 (as viewed in FIGS. 3 and 4) by means of a return spring 78 interposed between leg 70 and frame 10 so that leg 72 is normally in engagement with one tooth of its associated star wheel E. Thus, leg 70 of pawl G is normally displaced from star wheel E, as is best shown in FIG. 3. The plunger 80 of solenoid I is coupled to the leg 72 of pawl G by means of a coupling member 82 such that when the solenoid is energized the plunger is retracted, causing pawl G to pivot in a counterclockwise direction about post 76, as viewed in FIG. 3, to engage one tooth of its associated star wheel E. Similarly, leg 72 of pawl F is normally biased into engagement with its associated star wheel D by mans of return spring 84 connected between its leg 70 and support frame 10. The plunger 86 of solenoid H is coupled with leg 72 of pawl F by means of a coupling member 88 so that when the solenoid is energized plunger 86 is retracted to pivot pawl F in a clockwise direction about pivot post 76, as viewed in FIG. 3.

RATCHET WHEEL STOP MEMBERS In accordance with the present invention, the add-subtract counter is provided with means for continuously preventing rotation of each star-shaped ratchet wheel D and E in a direction opposite to that when it is driven by its associated pawl F or G. This means includes an add wheel stop member J and a subtract wheel stop member K. Members K and J are each loosely mounted at one end about a common pivot post 90 which extends between plate 22 and'leg:16. -A"tor sion spring 92 is coaxially wrapped about post 90 and is coupled between plate 22 and member .I so as to normally resiliently bias member] against itsassociated star wheel D. Similarly, a torsion spring 94 is coaxially wrapped about post 90 and is coupled between leg 16 and member K so as to normally resiliently bias member K against its associated star wheel E. Members J and K are each stepped down in two steps to provide two stop surfaces 96 and 98. As shown in FIG. 3, member K is resiliently biased in a clockwise direction about post 90 so that one tooth of its associated star wheel E normally rests on stop surface 96 preventing counterclockwise rotation of star wheel E. Similarly, as shown in FIG. 3, stop member J is normally resiliently biased in a counterclockwise direction about pivot post 90 so that one tooth of its associated add star wheel D normally rests on stop surface 96 of member J, preventing clockwise rotation of star wheel D.

ELECTRICAL CIRCUIT The electrical circuit for the counter is shown in the schematic circuit of FIG. 6 from which it will be noted that add solenoid I-I includes a solenoid coil 100 and subtract solenoid I includes a solenoid coil 102. Switch SW-l associated with add solenoid H is a single pole, double throw switch including a movable contact 104 and two stationary contacts 106 and 108. Similarly, switch SW4 associated with subtract solenoid I is a single pole, double throw switch including a movable contact 110 and two stationary contacts 112 and 114. Also, switch SW-3 is a single pole, double throwswitch having a movable contact 116 and a pair of stationary contacts 118 and 120. A load L is connected across contacts 116 and 120 to a suitable voltage source S. Add solenoid coil 100 is connected across an energizing voltage source V through contracts I04 and 106 of switch SW-l and a normally open switch X, which serves as an external add pulser. Similarly, solenoid subtract coil 102 is connected across voltage source V through contacts 110 and 114 of switch SW-2 and a normally open switch Y, which serves as an external subtract pulser. Cam follower 54 serves to actuate movable contacts 110 and 1 16 of switches SW-2 and SW-3 in such a manner that these contacts are in a position as shown when numeral wheels 28 and 30 display a 00 indication as is shown in FIG. 2. These two switches are transferred upon completion of one count of an addpulse as will be described hereinafter. Cam follower 66 serves to actuate movable contact 104 of switch SW-l in such a manner that contact 104 is in the position as shown in FIG. 6 except during the period that numeral wheels 28 and 32 display the numeral 99 at which time contact 104 is transferred so as to engage stationary contract 108.

OPERATION Initially, the counter is adjusted so that numeral wheels 34 and 30 display a count of 01 and numeral wheels 32 and 28 display a minimum count of 00. Accordingly, the movable contacts of switches SW-l, SW-2, and and SW-3, are initially positioned as shown in FIG. 6. Upon a momentaryclosure of external pulser switch X, an add pulse is supplied to coil 100 of solenoid H whereupon its plunger 86 is retracted. Leg 72 of add pawl F is displaced away from its associated star wheel D and leg 70 of pawl F engages one tooth of the star wheel and rotates it the first portion of a one step movement in a counterclockwise direction, as viewed in FIG. 3. At this point, return spring 84 pivots pawl F in a counterclockwise direction about post 76, as viewed in FIG. 3, so that leg 72 engages one tooth of star wheel D and completes the first one step movement of star wheel D. The degree of rotation of star wheel D is transferred through differential C so that the differential output gear 68 drives gear 24 on shaft B a distance sufficient that numeral wheel 28 is driven one step to display digit 1. Thus, each time add external pulser X momentarily closes, numeral wheel 28 is stepped one step to display a higher digit. Upon application of the tenth pulse, numeral wheel 32 is displaced one step through the tens transfer means including gear 36 so that numeral 32 displays the digit 1, for a count display of 10. When the count on numerals wheels 32 and 28 is 99 and the count on numerals wheels 34 and is 00, cam follower 66 of switch SW-I is located in indentations 60 and 64 in cam wheels 58 and 62, respectively, whereupon movable contact 104 of switch SW-l is transferred to engage stationary contact 108. Thus, additional add pulses supplied by external pulse X will not actuate add solenoid H. After the first add pulse has been received, cam follower 54 is on the periphery of cam wheels 46 and 50 so that movable contact 110 of switch SW-2 is in engagement with stationary contact 114 and movable contact 116 of switch SW-3 is in engagement with stationary contact 120. The load L is energized by source S and a circuit is completed for energization of subtract solenoid l. Thereafter, upon receipt of a subtract pulse, upon momentary closure of subtract pulser Y, subtract solenoid I is energized to rotate star wheel E in a clockwise direction, as viewed in FIG. 4, so that numeral wheel 28 displays one count less than it displayed prior to the receipt of the subtract pulse.

The operation of stop members J and K is illustrated with respect to member K in FIGS, 4 and 5. The solid lines in FIG. 5 illustrate the relative positions of stop member K and star wheel E prior to the energization of subtract solenoid I. It will be noted that one tooth of star wheel E rests on stop surface 96 of stop member K preventing counterclockwise movement of the star wheel, as viewed in FIG. 5. Upon energization of solenoid I, pawl G pivots in a counterclockwise direction about pivot post 76, as viewed in FIG. 4, so that its leg 70 engages one tooth of star wheel E and displaces the star wheel in a clockwise direction against the opposing resilient force exerted by stop member K. During this stroke, star wheel E is displaced from the position as shown by the dotted lines in FIG. 4 to the position as shown by the solid lines. Thus, at the completion of the stroke one tooth rests on stop surface 98 preventing counterclockwise rotation of star wheel E. Return a spring 78 then serves to pivot pawl G in a clockwise direction about pivot post 76 so that pawl 72 engages one tooth of star wheel E'to complete the one step rotational movement of st wheel E. During this completion stroke, star wheel E is displaced from the position as shown by dotted lines in FIG. 5, where one tooth rests on stop surface 98, to the position as shown by the solid lines in FIG. 5 where the same tooth rests on stop surface 96. Throughout the period that the star wheel is being displaced in response to one subtract pulse, it is continuously prevented from rotation in a counterclockwise direction by means of stop member K whether surface 96 or 98 opposes the star wheel tooth.

Reference is now made to FIG. 7 which is similar to FIG. 3 with the exception that stop member K has been deleted. The purpose of this figure is to illustrate the manner in which a miscount may be obtained with an add-subtract counter similar to that described in US. Pat. to M. Hegner No. 2,973,899, which does not include stop members J and K. Pawl G is shown by solid lines in the position obtained when subtract solenoid I is deenergized. Pawl G is also shown by dotted lines in the position obtained at a point in time after the solenoid I has been energized but prior to the time that leg engages one tooth of starter wheel E. Thus leg 72 is not in engagement with star wheel E and hence the star wheel is in a free-floating condition; that is, it may be rotated a slight distance in a counterclockwise direction until one tooth of the star wheel engages leg 72 at its displaced position. The amount of rotation permitted star wheel E is shown by the dotted lines of the star wheel in FIG. 7. This displacement is sufficient that when pawl G completes its counterclockwise stroke, leg 70 will miss the intended tooth on star wheel E; that is, leg 70 will not engage star wheel E to rotate the star wheel in a clockwise direction. This displacement of the star wheel, when in a freefloating condition, may result when the add solenoid H is energized just prior to the point in time at which the subtract solenoid I is energized. For example, leg 70 of add pawl F may be displacing star wheel D in a clockwise direction as viewed in FIGS. 3 and 7 at the point in time that star wheel E is in its free-floating condition. Since the load presented by star wheel E on the differential C is less than that presented by the numeral wheel assembly A on shaft B, star wheel E will be driven in a backward direction through the differential so that it is displaced to the position as shown by the dotted lines in FIG. 7, whereupon when leg 70 of subtract pawl G completes its solenoid driven stroke it will miss the intended tooth on the star wheel, resulting in a miscount.

A miscount, as discussed above with respect to FIG. 7, is eliminated by the inclusion of stop members .I and K in the add-subtract counter of this invention. Reference is made, for example, to FIG. 8 which is similar to FIG. 7 but includes stop member K. Again, pawl G is shown by solid lines in the position obtained when its solenoid I is deenergized and by dotted lines in a position corresponding to that shown in dotted lines in FIG. 7 obtained after the solenoid has been energized but prior to the point in time at which leg 70 engages one tooth of star wheel E. But, star wheel E is not left in a free-floating condition as in the case of FIG. 7 since one tooth of the star wheel rests on stop surface 96 of stop member K preventing counterclockwise rotation of the star wheel. Thus, with the inclusion of stop member .I and K it is seen that even if the add and subtract pulses be received slightly out of phase a miscount will not result.

The invention has been described in connection with a particular preferred embodiment, but is not to be limited to same. Various modifications may be made without departing from the scope and spirit of the present invention as defined by the appended claims.

We claim:

I. In an add-subtract counter for adding input add pulses and subtracting input subtract pulses and displaying the result by means of at least one numeral wheel carried by a driven shaft adapted for step by step rotation in opposing directions; said counter having an add ratchet wheel and a subtract ratchet wheel respectively driven step by step in opposing directions by an add pawl and a subtract pawl, each having a stroke of predetermined length, and difi'erential means drivingly coupling said ratchet wheels with said shaft, the improvement comprising:

stop means associated with each said ratchet wheel acting during the pawl strokes for continuously preventing rotation of each said wheel in a direction opposite to that when driven by its associated pawl; and,

each said stop means including a plurality of stopping surfaces, one of said plurality of said surfaces engaging its associated wheel during one portion of the associated pawl stroke and another of said plurality engaging the associated wheel during another portion of the associated pawl stroke.

2. The add-subtract counter as defined in claim I wherein each said pawl is U-shaped and includes first and second ratchet wheel driving ends for effecting said step by step rotation. said one of said stopping surfaces engaging its associated wheel until said first driving end is engaged with said wheel and said another of said stopping surfaces engaging its associated wheel when said first end reaches the end of its stroke and prior to engagement of said wheel by said second end.

3. In an add-subtract counter as set forth in claim 2 wherein each said stop member is resiliently biased into engagement with its said associated ratchet wheel.

4. In an add-subtract counter for adding input add pulses and subtracting the input subtract pulses and displaying the result by means of a numeral wheel carried by a driven shaft adapted for step by step rotation in opposing directions; said counter having an add ratchet wheel and a subtract ratchet wheel respectively driven step by step in opposing directions by an add pawl and a subtract pawl, wherein each said pawl is U-shaped having first and second ratchet wheel driving legs, said pawl being pivotally mounted and resiliently biased so that said first leg is normally not in engagement with said associated ratchet wheel and said second leg is normally in engagement with said associated ratchet wheel to prevent rotation of said wheel in a direction opposite to that when driven by said pawl, means for pivoting said pawl so that said second leg is displaced from said associated ratchet wheel and said first leg engages and drives said ratchet wheel an initial portion of a one step movement whereupon said pawl returns to its normal position and in doing so said second leg drives said ratchet wheel the remaining portion of said one step movement, and difierential means drivingly coupling said ratchet wheels with said shaft, the improvement comprising:

means for continuously preventing rotation of each said ratchet wheel in a direction opposite to that when driven by its associated pawl wherein said rotation prevention means includes an add wheel stop member and a subtract wheel stop member respectively resiliently biased into engagement with said add and subtract ratchet wheels; and each said stop member comprising a double check stop member having a first stop surface for said rotation prevention engagement with its associated ratchet wheel during the period said pawl is being pivoted from its normal position to its position when said first leg engages and drives said ratchet wheel, and a second stop surface for rotation prevention engagement with its associated ratchet wheel during the period that said pawl returns towards its normal position until said second leg engages said ratchet wheel and drives said ratchet wheel the remaining portion of said one step movement. 

1. In an add-subtract counter for adding input add pulses and subtracting input subtract pulses and displaying the result by means of at least one numeral wheel carried by a driven shaft adapted for step by step rotation in opposing directions; said counter having an add ratchet wheel and a subtract ratchet wheel respectively driven step by step in opposing directions by an add pawl and a subtract pawl, each having a stroke of predetermined length, and differential means drivingly coupling said ratchet wheels with said shaft, the improvement comprising: stop means associated with each said ratchet wheel acting during the pawl strokes for continuously preventing rotation of each said wheel in a direction opposite to that when driven by its associated pawl; and, each said stop means including a plurality of stopping surfaces, one of said plurality of said surfaces engaging its associated wheel during one portion of the associated pawl stroke and another of said plurality engaging the associated wheel during another portion of the associated pawl stroke.
 2. The add-subtract counter as defined in claim 1 wherein each said pawl is U-shaped and includes first and second ratchet wheel driving ends for effecting said step by step rotation, said one of said stopping surfaces engaging its associated wheel until said first driving end is engaged with said wheel and said another of said stopping surfaces engaging its associated wheel when said first end reaches the end of its stroke and prior to engagement of said wheel by said second end.
 3. In an add-subtract counter as set forth in claim 2 wherein each said stop member is resiliently biased into engagement with its said associated ratchet wheel.
 4. In an add-subtract counter for adding input add pulses and subtracting the input subtract pulses and displaying the result by means of a numeral wheel carried by a driven shaft adapted for step by step rotation in opposing directions; said counter having an add ratchet wheel and a subtract ratchet wheel respectively driven step by step in opposing directions by an add pawl and a subtract pawl, wherein each said pawl is U-shaped having first and second ratchet wheel driving legs, said pawl being pivotally mounted and resiliently biased so that said first leg is normally not in engagement with said associated ratchet wheel and said second leg is normally in engagement with said associated ratchet wheel to prevent rotation of said wheel in a direction opposite to that when driven by said pawl, means for pivoting said pawl so that said second leg is displaced from said associated ratchet wheel and said first leg engages and drives said ratchet wheel an initial portion of a one step movement whereupon said pawl returns to its normal position and in doing so said second leg drives said ratchet wheel the remaining portion of said one step movement, and differential means drivingly coupling said ratchet wheels with said shaft, the improvement comprising: means for continuously preventing rotation of each said ratchet wheel in a direction opposite to that when driven by its associated pawl wherein said rotation prevention means includes an add wheel stop member and a subtract wheel stop member respectively resiliently biased into engagement with said add and subtract ratchet wheels; and each said stop member comprising a double check stop member having a first stop surface for said rotation prevention engagement with its associated ratchet wheel during the period said pawl is being pivoted from its normal position to its position when said first leg engages and drives saiD ratchet wheel, and a second stop surface for rotation prevention engagement with its associated ratchet wheel during the period that said pawl returns towards its normal position until said second leg engages said ratchet wheel and drives said ratchet wheel the remaining portion of said one step movement. 